One of the greatest challenges for society in the 21st century is to meet the growing demand for energy for transportation, heating and industrial processes, and to provide the raw materials for the industry in a sustainable way. More importantly, the future energy and raw materials supply must be met with a simultaneous substantial reduction of green house gas emissions. The oil, gas, and coal industries have dominated not only the energy market for the past 80 years but also the carbon-based chemicals industry. Declining crude oil reserves, political unrest and increasing global demand of oil and its refinery products (petro-derived chemicals aka petrochemicals), which form the basis for the industrial chemicals sector have driven up the cost of energy as well as these key hydrocarbon derivatives and their downstream products. Further, the combustion and refinement of fossil fuels has sparked concerns regarding the effects of greenhouse gas emissions on global warming. Incited by concerns for the environment and recent advances in biotechnological research synthetic biology techniques can now be applied to engineering of microbes to enable biosynthetic (renewable) manufacture of important raw materials for the chemical industry. At present the production of ethanol as a liquid biofuel from renewable plant resources is the principal approach being taken.
Ethanol has already been introduced on a large scale in Brazil, the US and some European countries. Ethanol can be blended with petrol or used as neat alcohol in dedicated engines. Currently, ethanol for the fuel market is produced from sugar (Brazil) or starch (USA). The production of ethanol from starch-containing materials requires a liquefaction step (to make starch soluble) and a hydrolysis step (to produce glucose). The resulting glucose is readily fermented. However, this raw material base, which also is used for animal feed and human needs, will not be sufficient to meet the increasing demand for fuel ethanol; and the reduction of greenhouse gases resulting from the use of sugar or starch-based ethanol is not as high as desirable (Farrell et al., 2006).
To address both these limitations, the exploitation of lignocellulose feedstocks, such as agricultural and forest residues as well as dedicated crops, for the production of ethanol, has been explored. There are many techno-economic challenges facing the lignocellulose-to-ethanol process, reviewed in (Hahn-Hagerdal et al., 2006).
First, cellulose and hemi-cellulose have to be de-polymerized into soluble sugars by biodegradation. Enzyme conversion is substrate specific without by-product formation, which reduces inhibition of the following processes. However, enzyme catalyzed conversion of cellulose to glucose is slow unless the biomass has been subjected to pretreatment, which is also required to reach high yields and to make the process commercially successful (Mosier et al., 2005).
Second, the de-polymerization of cellulose produces a mixed-sugar hydrolysate containing six-carbon (hexoses) and five-carbon (pentoses) sugars which have to be efficiently fermented into ethanol, as well as fermentation inhibitory compounds—low molecular weight organic acids, furan derivatives, phenolics and inorganic compounds released and formed during pretreatment and/or hydrolysis of the raw material (Larsson et al., 2000). Lignocellulosic raw materials, in particular hardwood and agricultural raw materials, can contain 5-20% (or more) of the pentose sugars xylose and arabinose, which are not fermented to ethanol by the most commonly used industrial fermentation microorganism, the yeast Saccharomyces cerevisiae. Pentose fermenting microorganisms have been genetically engineered and methods are being developed to remove toxic inhibitors using chemical or physical methods, reviewed in (Hahn-Hagerdal et al., 2006).
Third, to minimize process energy demands, advanced processes to integrate de-polymerization of cellulose and fermentation of the resultant sugars must be developed. Two approaches have been taken: separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation of cellulose (SSF). In SHF, cellulose is first hydrolyzed to glucose and then glucose is fermented to ethanol. The primary advantage of SHF is that hydrolysis and fermentation occur at optimum conditions; the disadvantage is that cellulolytic enzymes are end-product inhibited so that the rate of hydrolysis is progressively reduced when glucose and cellubiose accumulate (Tengborg et al., 2001). In SSF, hydrolysis and fermentation occur simultaneously in the same vessel, and the end-product inhibition of the enzymes is relieved because the fermenting organism immediately consumes the released sugars. Furthermore, the fermentation seems to decrease the inhibition of enzymes by converting some of the toxic compounds present in the hydrolysate (Tengborg et al., 2001). This increases the overall ethanol productivity, the ethanol concentration and the final ethanol yield (Söderström et al., 2005).
For all the excitement surrounding the production and use of ethanol as a biofuel, there are a number of weaknesses to this approach. There is a need for methods to produce hydrocarbons that can be used as fuel more efficiently than ethanol. These methods also need to provide a means for substantially reducing green house gases.